1. Field of the Invention
This invention relates to a rotary electric machine having permanent magnets, or more in particular to a rotary electric machine with a plurality of magnetic poles comprising a plurality of permanent magnets of different B-H characteristics so as to increase the amount of magnetic fluxes required for the rotary machine and further prevent demagnetization as desired, where the demagnetization means that the magnetic field is weakened.
2. Description of the Prior Art
Permanent magnets are widely used in the field of rotary electric machines. Desirable characteristics of magnets used for the electric rotary machines include high magnetic flux density B and high coercive force or intensity of magnetic field H. Generally, in the rotary electric machine which utilizes the amount of magnetic fluxes, a high magnetic flux density B leads to the advantage of the possibility of a smaller magnet size to obtain the same amount of magnetic fluxes, thereby reducing the size and weight of the machine. Also, a high magnetic field intensity H, which has the relation to the demagnetization due to the armature reaction, contributes to prevention of changes in the rotary machine characteristics which otherwise might occur due to the decreased amount of magnetic fluxes in operation, thereby maintaining the predetermined characteristics.
However, there exists no magnet which is high both in magnetic flux density B and coercive force H. For example, the anisotropic alnico magnet has a low coercive force of 700 Oe in spite of its high magnetic flux density B of 13,000 gauss. On the other hand, the anisotropic strontium ferrite magnet with a high coercive force of 3,400 Oe is as low as 4,000 gauss in magnetic flux density B.
There is a need for selection between these incompatible features in actual operation of a rotary electric machine with magnets. When demagnetization is emphasized, for example, magnets with a high coercive force are used, with the result that the small size of magnetic flux density B makes it necessary to enlarge the size of the magnets to obtain a predetermined amount of magnetic fluxes, leading to the disadvantage of the bulkiness of the machine. In the event that a high amount of magnetic fluxes is preferred, by contrast, magnets with high magnetic flux density are employed in spite of the disadvantage that the low coercive force H necessitates the incorporation of an exciting coil for demagnetization, thus complicating the internal structure and at the same time giving rise to the need for restoration of magnetic characteristics by the occasional flow of current in the exciting coil.